T869 Climate change: from science to lived experience. Module 1: Introduction to climate change in the context of sustainable development. Textbook

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Module 1: Introduction to climate

change in the context of sustainable

development

TEXTBOOK

By Gordon Wilson, Victor Fairén, Javier García-Sanz, Ignacio Zúñiga, Daniel Otto, Helmut Breitmeir, Dina Abbott and Carolien Kroeze

T869 Climate Change: from science to lived

experience

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Disclaimer

This project has been funded with support from the European Commission. This publication reflects the views only of the author, and the Commission cannot be held responsible for any use which may be made of the information contained therein.

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Before you start: the aim and learning outcomes of this chapter 151 5.1 The interrelation of individual behaviour and social structures 151 5.2 Whose influence counts: individual agency or structure? 155 5.3 Mediating social relations: power, powerlessness and knowledge 157 5.4 Social context, embeddedness and climate change 159 5.5 Social embeddedness and consequences of climate chaos 161 5.6 Social embeddedness, inequality and climate change 162 5.7 Climate change as remapping equality and inequality 163 5.8 A place for sociology in debating climate change? 164

5.9 Conclusion to chapter 5 168

References for Chapter 5 of Module 1 170

6: Conclusion: integrating the perspectives within the paradigm of sustainable

development 172

6.1 The main messages of the disciplinary inputs to Module 1 172

6.2 Integrated Assessment Models 173

6.3 From integrated assessment models to sustainable development? 177

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Before you start: aims, learning outcomes and how to study this

module

The overall purpose of Module 11_{is to introduce the concept of climate change within a}

context of sustainable development.

The module asks the fundamental question: How can we evaluate what others tell us should be done about climate change in order to make our own reasoned judgements? It therefore has primarily an analytical rather than a normative focus. In other words, the module does not offer prescriptions of what should be done. Rather, it equips you to make sense of, and critically analyse, the prescriptions of others from a variety of perspectives. ‘Others’ may be literally anybody, from:

Politicians to concerned citizens;

Policy makers to practitioners who are charged with enacting policy;

International bodies such as the United Nations and European Union to governments at all levels;

Non-governmental organizations to private sector firms.

Learning outcomes

Learning outcomes concern what you should know, understand and be able to do on completion of a module. They are important indicators of your learning development. We recommend that you use them as a checklist of your progress as you work through Module 1.

After studying Introduction to climate change in the context of sustainable development you should be able to:

a) Demonstrate knowledge and understanding of:

(i) The perspectives on climate change causes, impacts and mitigation/adaptation possibilities from a range of sciences: natural science, economics, political science and sociology

(ii) The basic association of climate change with human energy requirements (iii) The impacts of climate change on natural resources, especially water, and

consequent effects on human welfare

(iv) The integration of different scientific perspectives on climate change through the concept of sustainable development.

b) Be able to:

(v) Examine critically a range of media and perspectives on climate change and sustainable development

(vi) Apply the concept of sustainable development to integrate a range of climate change perspectives

c) Apply the following key skills:

1 There are two other modules in this series. Module 2 is The lived experience of climate change. Module 3 is Interdisciplinary methodologies for investigation into the ‘lived experience’ of climate change. A Water case study is also provided as an extended text. These other modules and the Water case study might be referred to from time to time in this e-textbook and corresponding e-workbook.

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(vii) Interpret scientific, statistical and other data

(viii) Search for and make judgements on evidence from a range of sources (ix) Marshall evidence, and develop and communicate in your own words an

argument.

(x) Construct knowledge on climate change through communicative exchange with others and develop transboundary competence.

How to study this module

As with other teaching modules in this series, Introduction to climate change in the context of sustainable development consists of a ‘textbook’ comprising a central narrative about the subject, a ‘workbook’ containing a series of activities for you to perform, and a detailed case study on water and climate change. The ‘textbook’ follows on in this document. It is like a conventional book, although being in a virtual learning

environment it may refer to a range of media and not just the printed word. Once you have read through the textbook carefully you should be able to meet the ‘knowledge and understanding’ learning outcomes above.

The ‘workbook’ is contained in a separate document, and again it may refer to a range of media. The ‘workbook’ helps you reach a more extensive and deeper, critical

understanding of the subject matter. It does this in two complementary ways, by providing: you with:

 Further reading and audiovisual links, and asking you to search yourself for additional sources.

 Opportunities to develop through practice the ‘be able to’ skills (which we call cognitive or thinking skills in relation to the subject) and the ‘key skills’ (skills which are transferable across a range of subjects) above.

Thus, although, with one possible exception2_{, the choice is ultimately yours, we}

recommend that you do not neglect the workbook and its activities. Your sense of overall satisfaction with the module is likely to be greater if you engage with them. Also,

although the textbook may refer directly to the water case study, the purpose of this case study is for you to apply critically the principles and concepts of the module to a real-world challenge associated with climate change. One or more of the workbook activities will help you do this and therefore the workbook is the main point of reference for the water case study.

How in practice might you combine the three main resources at your disposal in this module – the textbook, workbook and water case study? You should choose the method which best suits your own learning style. One way is to go to workbook activities at the points that they are indicated in the textbook. Another way is to read the whole of the narrative in the rest of this textbook (and the water case study), without worrying too much about remembering the detail. Then, having completed your reading, work through the activities in the workbook systematically, analysing sections of the narrative and water case study again more closely as appropriate.

2 The exception concerns any workbook activities which might be deemed compulsory by your accrediting institution. The obvious example concerns workbook activities which are designed for group work. If the key skill of transboundary competence or similar formulation is part of the learning outcomes of the accrediting institution, satisfactory participation in activities that deliver that learning outcome is likely to be a requirement.

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1. Introduction to Module 1

By Gordon Wilson, Víctor Fairén and Carolien Kroeze

The subject of this module, Climate Change within Sustainable Development, is situated in an area in which natural science, and more recently other disciplinary subjects in the social sciences, must address a problem that modern society and technology have created in part.

There is some confusion between weather and climate and it is best to address this at the outset. Although they both apply to how the atmosphere behaves, they are not

synonymous. Weather is the condition of the atmosphere that prevails at a given moment and at a given place which is perhaps a few kilometres across, and may change within days or even hours. It is then correct to state weather is sunny and warm today and will be rainy and cold tomorrow. The chaotic nature of weather makes it unpredictable beyond a few days and it is the reason why weather forecast does not extend beyond a week or so.

In spite of the irregular behaviour on a daily basis, observations show that there is some long-time regularity in the behaviour of the atmosphere. We define, for example, a Mediterranean climate, with mild winters and hot, dry summers because along the years those have been the general atmospheric conditions ruling the Mediterranean basin. This is what is termed as ‘climate’, the atmospheric character of some geographic area, shown by records over many years. Climate is then defined as the average weather, generally over a period of thirty years. Atmospheric variability on timescales of months or longer is known as climate variability, and statistics relating to conditions in a typical (as opposed to a particular) season or year are referred to as climatological-mean statistics.

Projections in climate (long-term average weather) are more manageable than their counterparts in particular weather predictions.

When speaking about climate and its variability (whether “natural” or not), we mean a highly dynamic system in whose description the equations of physics play a fundamental role. Therefore, it is of interest to natural scientists. But it is also an issue that has always transcended the boundaries of science and involves perspectives that derive from the fields of economics, politics, sociology or cultural and religious beliefs. As long as it is so, issues regarding climate are the subject of debate and disagreement, in part because science does not meet the expectations that society demands from it, but also because we weigh against one another different ways of understanding science and scientific

knowledge and, above all, because it is interpreted in the light of the diversity of beliefs, values, attitudes, aspirations and behaviour. This problem is not unique to climate change. Biomedicine, ecology, resource management, just to name a few, are all topics where it is the same: their objects of study are complex, scientific understanding is limited and they have a profound impact outside the field of science.

Societies have always sought security in both climate and the supply of basic resources: water, food and energy. And this is because this security is a prerequisite both for meeting fundamental human needs and for economic growth and development. Climate and those basic resources are all intimately tied together. Food and water are dependent on both climate and energy. Before the industrial revolution, energy derived primarily from animal and manpower (which relied on an adequate supply of food and water) and wood as fuel. The intensive use of fossil fuels (carbon, oil and gas) has altered the equation for a number of reasons. We can basically underline four:

1) For almost two hundred years, we have been returning to the atmosphere billions of tonnes of carbon as carbon dioxide, which were fixed long ago by plants and plankton and later deeply buried, thus reversing a process that has led to the

conditions which have witnessed the development of Nature and human societies as we know them today.

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2) There is strong scientific evidence in favour of a possible climate alteration due to the release of such amount of carbon dioxide and other greenhouse gases. How large this shift in climate will be is uncertain and maybe scientific knowledge will never be able to provide a definite answer. In spite of that, what it is feared from our

knowledge of past climate events is the possibility of a large shift due to the highly nonlinear nature of climate processes – yet unknown feedbacks might give rise to large unexpected consequences.

3) This eventual climate modification raises the suspicion of a non-negligible impact on our food and water supplies, among other consequences.

4) There is no foreseeable end to our dependence on burning fossil fuels. Other sources of energy are not yet technologically and economically competitive. If we are to meet the growing demand of populations that will presumably reach ten thousand million in a few years from now, there is no apparent alternative in sight that is able to supply such cheap and abundant sources of energy as the otherwise dwindling fossil fuels do.

Set up jointly in 1988 by the United Nations Environment Programme and the World Meteorological Organisation, the Intergovernmental Panel on Climate Change (IPCC) concluded in its fourth Scientific Assessment that most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations [IPCC, 2007b]. The question now concerns what will happen in the future? This question is not easily answered.

The future of our climate is uncertain. This uncertainty is related to uncertainties about future trends. There are many individual causes of climate change, including fossil fuel combustion as noted above, deforestation and land use change leading to a range of greenhouse gas emissions of which carbon dioxide (CO2), methane (CH4) and nitrous

oxide (N2O) are the most important. Exploring the future, therefore, requires projections

of all these activities and processes. In addition, as you will see, the climate system is very complex with many feedbacks. We know the system to some extent, but many uncertainties remain making it difficult to quantify the local impacts of future climate change. Moreover, appropriate climate policies are not easily identified, and range from adapting to climate change impacts to mitigation measures aimed at reducing greenhouse gas emissions. Finally, there are many stakeholders involved and the stakes are high. Module 1 illustrates this complexity. It discusses the science of climate change, as well as economic, political and societal aspects. The different chapters in Module 1 are written from different disciplinary perspectives.

Despite this complexity, the question how to approach climate change is being addressed by scientists. This is done in so-called integrated assessments. Such assessments apply the judgment of experts to existing knowledge to provide scientifically credible answers to policy-relevant questions [Leemans, 2008].

The IPCC Scientific Assessments are among the most widely used and most credible in this regard. The fourth IPCC Scientific Assessment was published in 2007, and includes a synthesis report [IPCC, 2007a], and summaries for policy makers [IPCC, 2007c]. We suggest that you read these two documents for an overview of the current state of knowledge on climate change. The IPCC scenarios indicate an increase in the global temperature by 1–6 degrees Celsius by 2100, which may considerably alter precipitation patterns and sea levels.

Thus, it is in this context of what is both known and what is uncertain for the future, that anthropogenic (human-induced) climate change has emerged as a global challenge in recent decades because of its potential to disrupt dominant models of human

development, lives and livelihoods in affluent and poor countries alike across the world. Proximate explanations of climate change are to be found in the natural sciences. Human

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contextual explanations are to be found in a) the dominant model of economic development and its fundamental dependence on energy; b) the challenges facing political leaders and policy makers to agree and enact courses of action for mitigation and adaptation. While human beings impact on climate change, mainly through their energy requirements, climate change in turn impacts on them, their lives and livelihoods. Such impacts, however, are felt unevenly across human societies, being mediated by social relations of wealth, power and inequality.

The combination of these human factors is often integrated into the concept of

sustainable development, which seeks a model of economic development that does not irreparably harm the natural environment on which it depends, and which is equitable and just for both future and current generations. This leads to the well-documented three pillars of sustainable development – environment, economy and society. The relation of climate change to sustainable development and the integration of economic and social dimensions with those of the natural environment are illustrated by recent holistic practices of natural resource management.

The following chapters in this module develop the above skeletal argument: Chapter 2 covers the science of climate change;

Chapter 3 covers the economics, including economic and technological approaches to mitigating climate change and their importance when making policy;

Chapter 4 overlaps with Chapter 3 as the economics always spawns a politics. It

addresses the geopolitics of climate change: the international institutional setting and the factors that influence the effectiveness of global governance.

Chapter 5 examines the social impacts of climate change, and how these impacts are mediated by social relations of power and inequality.

Chapter 6 is the module Conclusion. It brings together Chapters 2, 3, 4 and 5 within the broad idea of sustainable development, while providing a critical examination of the concept. This final chapter also draws together issues of ‘scale’ and inter-scalar

connections. The inquiry in Chapter 2 on the science is necessarily focused on the planet Earth, while that in Chapters 3 and 4, on the economics and politics of climate change respectively, is conducted both at national and international scales. In contrast, Chapter 5 is at the local scale as it examines the impacts on people and communities. Thinking of these scalar dimensions raises two basic questions for Chapter 6. How does the national and international affect the local? How does the local affect the national and

international? The module ends by briefly examining these questions.

You might like to end this introductory chapter by reflecting upon its key message and on the usefulness of locating climate change within a context of sustainable development. If you undertake this reflection now, it will provide a useful overview within which you can locate the rest of Module 1. Activity 1.1 in the module workbook provides specific guidance and a discussion.

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2. What science tell us about climate change

By Víctor Fairén, Javier García-Sanz, Ignacio Zúñiga (‘Before you start’ is by Gordon Wilson, Module 1 coordinator, with Figures 2.1 and 2.2 supplied by Victor Fairén)

Before you start: the aim and learning outcomes of this chapter

Chapter 2 aims to introduce you to the science of climate change.

After studying Chapter 2 you should be able to understand:

1) The Earth’s climate as being driven by an energy input-output system, which originates with the energy received from the sun.

2) The concept of feedback in its many dimensions and its importance in analysing, and attempts to model, climate.

3) The mechanisms by which climate changes over both long and shorter time periods. 4) Especially the role in climate change of forcing agents, both natural and those

deriving from human activity.

5) In basic terms, the main models that have been developed to simulate climate, how it changes historically, and how it is predicted to change.

I am introducing this ‘Before you start’ to chapter 2 on behalf of Victor Fairén, Javier García-Sanz and Ignacio Zúñiga from the Universidad Nacional de Educación a Distancia (UNED) in Spain. I write as someone who was trained originally in natural science (chemistry) and moved later to social sciences. I have, therefore, some

experience of the challenges that non-scientists face when they try to understand natural science, and vice versa.

Natural science can be difficult for non-scientists, in a way that doesn’t operate with respect to social science. Most people have an everyday knowledge of social science concerns. Some they confront or experience directly (for example, when the national economy is doing well or badly and it impacts on them and their livelihoods), and/or others through what they read through the media or internet. Such commonsense knowledge has its challenges when students have to face the rigour of formal social science study. However, at least it provides a tacit ‘hook’ that enables one to engage. Natural science for non-(natural) scientists, however, seems much more distant and abstract. It works through formulae and equations which can appear to bear little relation to the world around us.

A key challenge concerns scale. The social sciences are by definition at a human scale. This might be at a big human scale when social scientists investigate, say, the global economy or a global issue such as the socio-economic impacts of climate change.

Equally, however, social science investigation is at a small scale, as when anthropologists do detailed studies among relatively small groups of people to unearth their livelihoods, social structures and relations between them. This (big or small) human scale facilitates common-sense understandings, even among lay people who also have an awareness of the connections between scales – how globalisation and the economic rise of China, for example, affect our own livelihoods and lifestyles.

Natural science, in contrast, can be, and is often, carried out at scales beyond our human imagination – both big and small. Take measurement. Size can be extremely small and measured by nanometres where a nanometre equals one billionth of a metre, and very big, for example gigatonnes, where a gigatonne equals one billion tonnes. Another example, which is crucial for our understanding of climate, is the scale of time. Social historians might go back several hundred years, but this is nothing compared to the geological timescales on which climatologists operate, which go back in time a billion years.

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Despite these and other challenges, it is important for scientists and non-scientists alike who are interested in climate change to have a basic understanding of the natural science that lies behind it. One good reason for this is that this science remains uncertain in many respects. This does not only concern the debate between those who believe that

anthropogenic (human) activity is largely responsible for global warming and those who are sceptical, but also within the former group there remains much debate about the rate and extent of warming and its impact on climate. Yet, in spite of, and perhaps even because of, this uncertainty policy makers draw on different aspects of the scientific evidence to make a case for whatever it is they are advocating. It is important then for anybody who is interested in climate change – both natural and social scientists -- to understand the evidence that is presented and be able to judge it.

Although I have presented understanding science as a challenge for non-scientists, do not despair if you fall into this group. Victor and his colleagues have produced this chapter of the module in a manner that should allow a good basic engagement with the subject, and don’t be put off if you don’t understand everything. It’s more important that you grasp the overall picture of the ways by which scientists themselves seek to understand our climate. The activities in the chapter 2 section of the Module 1 workbook are designed to test your grasp of the overall picture. If you can make a reasonable attempt at these activities, you have nothing to worry about.

Repeatedly this chapter refers to the different geological periods which, as noted above, stretch back in time up to a billion years. Within the big periods there are smaller periods of interest and within the smaller periods there are ‘events’ which themselves can last many years. Figure 2.1 summarises this timescale, and you may wish to refer to it as necessary as you go through the module.

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Another useful chart which you might wish to view repeatedly concerns the first two layers of the atmosphere. These are continually referred to throughout chapter 2. This is Figure 2.2.

Figure 2.2 The first layers of the atmosphere

2.1 Introduction to the science of climate change

Some 20-30 years ago, people started hearing about global climate and its troubles. For scientists, however, the problem has a long history. In the early 19th century already, Joseph Fourier seems to be the first in assessing the role of the atmosphere in acting as a greenhouse factor. Thanks to it the Earth temperature is higher than it would be in its absence. Some ten years later, another French scientist, Claude Pouillet, pointed at water vapour and carbon dioxide as the main greenhouse gases. We must wait until the end of that century in order to see a Swedish chemist, Svänte Arrhenius, a Nobel Prize winner for his work in electrochemistry, formulate quantitatively these ideas. Not only was Arrhenius aware of the possible negative effect of the gases emitted by the burning of coal by the industry of his time, but he went a step further and calculated this effect on temperatures. He concluded that the mean global temperature might rise several degrees. By the 1930s, global warming was already a reality although most scientists invoked some natural cycle to explain it. There was, however, a discordant voice. Guy Stewart Callendar, a British engineer, insisted on a link between human-induced carbon dioxide and global warming. He thought, however, as all his contemporaries did, that global warming would even be beneficial.

In the late 1950s, the introduction of new tools – computers and global monitoring systems – allowed scientists to address the climate issue by launching specific

international programmes. In the last fifty years, their results are at the root of society’s awareness. One major step was the issuing of a formal declaration at the United Nations Earth Summit, in 1992, according to which all signatories agreed on the Climate Change Convention, expressing the determination of stabilizing greenhouse gases concentrations at a level precluding any dangerous anthropogenic (i.e. human) perturbation of the climate system. Specific measures to implement this declaration were initially adopted in 1997 in Kyoto and entered into force on February 16, 2005.

At the end of 2012 the Kyoto Protocol runs out. On December 2009, a U.N. Convention on Climate Change took place in Copenhagen. Despite some good will, it came to naught. The accord, which falls short of a binding treaty sought by many nations, sets a goal of limiting global warming to below 2 degrees Celsius (3.6 Fahrenheit) above

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pre-industrial times. But it leaves each nation to set its own targets for 2020. It is even less mandatory than the Kyoto Protocol.

It is clear that the implementation of effective measures aimed at counteracting the present global warming cannot be made without the support of the World’s citizens, fully aware of the impending threat to the planet. This is presently lacking. In the current mood of “business as usual” the fulfilment of this goal has become a long-term contest of unpredictable outcome.

The scientific community is one of the participants. It has the privileged position of being one of the contributors to the generation of the needed knowledge for completing a picture of what awaits us. However, it must not only be right in its conclusions, but it must also learn how to argue its case and, above all, be convincing. It is not an easy task when we realise that we are confronted by a very difficult case. Climate is indeed a very complex system, but in spite of its persistent uncertainties, climate science and in particular climate change science is solidly grounded. In spite of that, a complete and comprehensive assessment of climate is surely an unreachable target: climate has too many intrinsic uncertainties to deal with. However, life is fundamentally uncertain and people are conscious that decisions must always be made in a context of less than a hundred percent assurance. The role of science is then that of unloading all reasons to make its case and convince despite all uncertainties. It is hoped that the present text will encourage the reader to participate in the achievement of this objective.

2.2 Global Warming

2.2.1 Escalating temperatures

Upon examining Figure 2.3 we realise that global temperatures have indeed increased over the last 120 years, in spite of year to year, or even decadal decreases, as happened in the period 1940–1960.

Figure 2.3 Graph of global annual surface temperatures relative to 1951–1980 mean temperature (Air and ocean data from weather stations, ships and satellites). Source: http://data.giss.nasa.gov/gistemp/graphs/(accessed 6 March, 2012)

Instrumental measurements starting around the middle of the 19th century plus the extrapolations made on values of temperatures in previous centuries lead to the

conclusion that never in the last millennium have temperatures shown such a high rate of increase. But temperatures and mean precipitations do not only go up but also show more variability over the planet in the last one hundred years. Important differences between land and sea, between different regions, between seasons, or even between day and night,

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constitute a natural phenomenon. What characterises the last hundred years is that those differences are accentuating and becoming largely superior to the statistical variability. Winters are warming faster than summers. This is particularly striking in Eastern and Southern Europe, where the number of cold days has notably reduced while days with suffocating temperatures are more frequent (See Figure 2.4).

Figure 2.4 Linear trend of seasonal March-April-May (MAM), June-July-August (JJA), September-October-November (SON) and December-January-February (DJF) temperature for 1979 to 2005 (°C per decade). Grey areas indicate insufficient data. Source: IPCC Fourth Assessment Report, chapter 3: Observations: Surface and Atmospheric Climate Change. http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch3s3–2–2–7.html(accessed 6 March 2012)

This global warming is not only detected in thermometer readings all over the world. There is much evidence coming from other sources as the following sub-sections illustrate.

2.2.2 Precipitations

Evaporation goes hand in hand with the scaling up of temperatures. It accounts for the 2 percent rise in precipitations that has been observed over the last one hundred years.3_But

this is not at all good news because the apparently beneficial effect is mitigated by the large regional differences (See Figure 2.5). The IPCC states in its 4th report (2007, Chapter 3: 262):

“Observations show that changes are occurring in the amount, intensity, frequency and type of precipitation. These aspects of precipitation generally exhibit large natural variability, and El Niño and changes in atmospheric circulation patterns such as the North Atlantic Oscillation have a substantial influence. Pronounced long term trends from 1900 to 2005 have been observed in precipitation amount in some places: significantly wetter in eastern North and South America, northern Europe and northern and central Asia, but drier in the Sahel, southern Africa, the Mediterranean and southern Asia. More precipitation falls now in the form of rain instead of snow in northern regions. Widespread increases in heavy precipitation events have been observed, even in places where total amounts have decreased. These

3 IPCC Fourth Assessment Report 2007, 0. See also Chapter 3, p. 254.

http://www.grida.no/graphicslib/detail/precipitation-changes-trend-over-land-from-1900-to-1994_3f8d (accessed 6 March 2012)

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changes are associated with increased water vapour in the atmosphere arising from the warming of the world’s oceans, especially at lower

latitudes. There are also increases in some regions in the occurrences of both droughts and floods”.

Figure 2.5 (Top) Trend of annual land precipitation amounts for the period 1901 – 2005; (bottom) for the period 1979 – 2005. Source: IPCC Fourth Assessment Report, chapter 3 Observations: Surface and Atmospheric Climate Change

http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch3s3–3–2–2.html(accessed 6 March 2012)

2.2.3 Oceans are also slowly warming up

Oceans levels and temperatures have been also rising since the end of the 19th century and the trend has been accelerating, starting in the early 1990s. In some locations, part of this change may be due to natural causes. Nevertheless, the alterations observed at the planetary scale cannot but be connected to global warming. Instrumental readings lead to an estimate of 0.6ºC for the global increase since 1860 [Folland et al., 2001]. However, that warming is unevenly distributed. Ocean dynamics and local conditions induce differences in warming among the different seas. Figure 2.6 displays those differences in seas contiguous to continental Europe. The North Atlantic has been warming less than the North and Baltic Seas, due to influences from the Arctic and the Ocean Conveyor Belt (See further ahead).

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Figure 2.6 Sea surface annual average temperature anomaly for period 1870–2006 with respect to 1982–2006 mean in different European seas. Source: European Environment Agency,http://www.eea.europa.eu(accessed 6 March 2012). Figure link:

http://www.eea.europa.eu/data-and-maps/figures/sea-surface-temperature-anomaly-for-period-1870–2006(accessed 6 March 2012)

Figure 2.7 Satellite altimetry measurements of average sea level rise for the period 1994 – 2004. Red solid line: 60-day average. Black solid line: trend. Source: Nasa Earth

Observatory, http://earthobservatory.nasa.gov/IOTD/view.php?id=6638 (accessed 6 March 2012)

Sea levels have increased up to 25 cm in some places (Figure 2.7). The extent of the phenomenon varies locally, as happens with tides, and depends on many factors, such as sea floor topography, irregularity of the coast line, land subsidence (as in Bangladesh) or isostatic land rises (see, for example, the isostatic rebound4_{of the Northern Baltic from} the last Glaciation). Narvik, in Northern Norway, registers a 3 mm rise per year, while

4 Isostatic rebound is a physical process by means of which a floating body rises when freed from a load. When a thick ice cap covered Scandinavia, the glaciers’ tremendous weight “sank” the lithosphere (earh’s crust) into the viscous upper mantle. Upon deglaciation, the buoyant force exerted by the upper mantle tends to re-establish equilibrium by raising the lithosphere. This process is extremely slow and still continues in the Scandinavian Peninsula, as well as in other parts of the World.

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Marseille, on the French Mediterranean Coast, 1 mm/year. Surface water expands as a result of climbing temperatures. Expanding sea water then readjusts itself vertically as it is constrained by the continental limits of oceanic basins. Moreover, melting continental ice packs contribute further to ascending sea level, but by a much smaller amount.5 As shown in Table 2.1 other sources different from thermal expansion contribute to rising sea-level. We can observe that the trend has gained momentum since the early 1990s. New satellite measurements give it a global dimension that cannot but amplify in the future. Due to its thermal inertia sea water has been warming much less than the atmosphere. That means that even if temperatures are someday stabilised, the oceans will continue to show noticeable effects, and, in particular, they will continue their expansion.

Table 2.1 Observed sea-level rise, and estimated contributions from different sources (in mm yr-1_{). Source IPCC 2007}

Source of sea-level rise 1961–2003 1993–2003

Thermal expansion 0.42 ± 0.12 1.6 ± 0.5

Glaciers and ice caps 0.50 ± 0.18 0.77 ± 0.22

Greenland ice sheet 0.05 ± 0.12 0.21 ± 0.07

Antarctic ice sheet 0.14 ± 0.41 0.21 ± 0.35

Sum of individual contributions 1.1 ± 0.5 2.8 ± 0.7

Observed sea-level rise 1.8 ± 0.5 3.1 ± 0.7

Difference 0.7 ± 0.7 ± 1.0

2.2.4 Shrinking ice packs

Most glaciers in the World are melting. In fact, the process has been particularly marked since the beginning of the 20th century (Figure 2.8). In the course of the last hundred years Mounts Kenya and Kilimanjaro, for example, have lost 92 percent and 82 percent of their glaciers, respectively [Kaser et al.,2004]. If the shrinking of African glaciers may be linked to local variations in the water cycle, no consensus exists on the reasons for similar observations of retreating glaciers in other parts of the planet. The melt is too fast in order to invoke global warming as the ultimate cause. As a matter of fact, the melt was already apparent well before global warming started to be significant (Figure 2.9).

5 A complete treatment of the issue may be found in: Sea Level Rise: History and Consequences, Edited By B.C. Douglas, M.S. Kearney and S.P. Leatherman, International Geophysics Series, Vol. 75, Academic Press, San Diego, 2001.

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Figure 2.8 Pictures of a melting glacier (Pedersen Glacier Kenai Fjords National Park, Alaska) in 1920 and 2005. Photo composition: Robert A. Rohde. Source: Global Warming Art,http://www.globalwarmingart.com/wiki/File:Pedersen_Glacier_jpg(accessed 6 March 2012).

Figure 2.9 Large-scale regional mean length variations of glacier tongues. The raw data are all constrained to pass through zero in 1950. The curves shown are smoothed. Glaciers are grouped into the following regional classes: SH (tropics, New Zealand, Patagonia), northwest North America (mainly Canadian Rockies), Atlantic (South Greenland, Iceland, Jan Mayen, Svalbard, Scandinavia, European Alps and Asia (Caucasus and central Asia). Source: IPCC Fourth Assessment Report, chapter 4: Observations: Changes in Snow, Ice and Frozen Ground (p.357).

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In any case, less snow is now falling. During winters (Northern and Southern

Hemispheres), nearly one third of the planet is covered with snow. The surface covered by snow in the Northern Hemisphere has diminished by 10% since the 1960s6_{and the}

snow-covered period is diminishing between 7 and 10 days per decade. Spring comes earlier and the arrival of autumn snow is delayed.

It has been observed that a substantial part of Arctic ice is being lost, despite

considerable year-to-year variability (see Figure 2.10). Its surface is estimated to suffer the loss of some 30,000 km2_{/year (approximately 2.9% per decade), essentially during}

summer.7

Figure 2.10 Trends in Arctic sea ice extent in March (maximum) and September (minimum) in the time period of 1979–2008. Cartographer: Hugo Ahlenius. Source: http://maps.grida.no/go/graphic/trends-in-arctic-sea-ice-extent-in-march-maximum-and-september-minimum-in-the-time-period-of-1979–20(accessed 6 March 2012).

2.2.5 The energy budget

Why are temperatures climbing? Why is this so worrying if variability is intrinsic to climate? Variability is a notorious fact today as well as in the history of climate (see Section 2.3 for a deeper treatment). Past climatic events have left their mark on the Earth’s surface. Research on these footprints has led us to realise that climate has undergone many changes during the Earth’s history. With sophisticated and complex techniques, scientists have been able to proceed to a reconstruction of the history of the climate of our planet. The reconstruction of the recent past is certainly more accurate and reliable than that of the distant past, being related to the availability of pieces of evidence where transformations of the Earth’s crust remove with time this evidence.

Paleoclimatology has nevertheless allowed us to have a pretty good image of the last 800,000 years, essentially thanks to air bubbles trapped in the Greenland and Antarctic ice packs, while little is known of how climate was and evolved 500 million years ago. What we know is nonetheless more than enough to convince us of the existence of both colder and hotter periods than the present – both types thriving with life – and also of transitions between those extreme types.

What is singular to the present rise of temperatures is that there is a consensus of the scientific community linking it specifically to human activities, particularly, to the

6_{http://www.ncdc.noaa.gov/indicators/} 7

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burning of fossil fuels. On the other hand, it poses a threat not to life itself but to “life as we know it now, including our present civilisation.”

So the initial question is changed to the following one: why are human activities behind the temperature switch? The answer lies in the modifications brought about by the flow of energy throughout the climate system by gases emitted from burning fuels: the so-called greenhouse gases. But before turning to those disrupting agents we must

understand how the planet conveys and transforms the energy it receives from the Sun.

Figure 2.11 Estimate of the Earth’s annual and global mean energy balance. Source: IPCC Fourth Assessment Report, chapter 1, Historical Overview of Climate Change Science. http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-1–1.html(accessed 6 March 2012). The flows of energy to which we refer to are sketched on Figure 2.11. According to the latest estimations, the Earth receives at the top of the atmosphere (TOA), in the form of shortwave radiation,8_{a mean incoming solar radiation of 341.3 W/m}2_{(watts per square}

metre. The watt is the basic unit of energy -- the joule -- per second). Part of it (totalling 101.9 W/m2_{) is directly returned by both the atmosphere and the planet’s surface.}

Consequently, 341.3 W/m2_{– 101.9 W/m}2_{= 239.4 W/m}2_{constitutes the absorbed solar}

radiation (ASR) by the planet, which sets the atmosphere and oceans into motion and thus defines climate. As a matter of fact, this absorbed energy follows a course of transformations and of complex exchanges between the climate agents. Figure 2.11 outlines the major features of this energy flow.9

This energy cannot remain on Earth. Otherwise, the planet would be heating up to untenable levels. In fact, it is returned to the outer space in form of long wave radiation – outgoing (infrared) long wave radiation (OLR). As shown in Figure 2.11, it is estimated that at the TOA the outgoing radiation is 238.5 W/m2_{. Let us compare to the figure of}

239.4 W/m2_{for the ASR: ASR – OLR = 239.4 W/m}2_{– 238.5 W/m}2_{= 0.9 W/m}2_{. This}

means that 0.9 W/m2_{are not returned to the outer space: they have been captured by the}

Earth’s atmosphere. The result is clear: The Earth is heating up and thus temperatures are rising.

8 Roughly speaking, shortwave radiation extends in the visible and near-visible range (0.4–4.0 μm in wavelength)

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2.2.6 Radiant energy and temperatures

We can now address the fundamental difficulty, which is that of trying to understand what is behind the energy imbalance and the ensuing temperature rise. We have been talking of incoming and outgoing short wave and long wave radiation. What do we mean by that? Energy is known, and traded, in a number of different forms. We have probably all heard about the epithets thermal, mechanical, chemical, electrical, elastic, nuclear, and so on, when referring to energy. The Earth, taken as a whole, which means including its atmosphere, exchanges energy with the outer space almost exclusively through a single form: radiant or light energy, that is, in the form of electromagnetic waves. This radiant energy is intrinsic to all bodies whose temperature is above the absolute zero on what is called the Kelvin (K) scale (0ºK which = –273ºC). For the simple fact of having a temperature above the absolute zero, a body emits light in a way which has been known to physics since the 19th century: the black body radiation. Moreover, it turns out that black body radiation provides us with a set of very precise working equations that relate the temperature of an object to the light it emits.

Figure 2.12: Displacement of the blackbody radiation spectrum with decreasing

temperature (in ºK). Hotter objects emit preferably in the short wavelength window and, as a body becomes cooler its emission shifts to longer wavelengths (lower frequency and less energetic light). Source: Education Resources of the Hong Kong Space Museum,

http://www.lcsd.gov.hk/CE/Museum/Space/EducationResource/Universe/framed_e/lecture/c h05/ch05.html(accessed 6 March 2012)

A blackbody does radiate energy at every wavelength although the intensity at which a given wavelength is emitted depends on the temperature of the body. If we draw the intensity at which a wavelength is emitted in terms of wavelengths we obtain a bell-shaped, universal curve: the theoretical black body curve at a given temperature, also known as the black body spectrum.10_{The graphs on Figure 2.12 depict four examples at}

different temperatures (in degrees Kelvin). We observe that the peak decreases in magnitude and shifts its position to the longer wavelengths region as the temperature lowers. It means that the light emitted by very hot bodies (> 7,000 K) is essentially ultraviolet light (short wave radiation). As the body becomes colder (in the range 4,000 K – 7,000 K) most radiant energy is in the form of visible light, or infrared light (long wave radiation) for temperatures below 4,000 K.

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The temperature of the outer, visible part of the Sun (the photosphere) is approximately 5,700 K. The peak is consequently in the visible range. This is why we see the Sun shine. On the other hand, the mean temperature of the Earth is 288 K (15 ºC). The Earth

radiates in the infrared range. This is why our planet does not “shine”, though that does not mean that it does not radiate light: its light is simply not visible; it is instead sensed as plain heat.11

We can now recall Figure 2.11. The outgoing long wave radiation (OLR), which was estimated to be 238.5 W/m2, is essentially infrared radiation. We are talking about infrared radiation and it is at this stage that the greenhouse gases come into action. We shall deal with who they are and what they do in the next few sections.

2.2.7 Atmosphere effects

Not all of the solar radiation received at the outer limits of the atmosphere reaches the surfaces of the Earth. The atmosphere selects various components of solar radiation. As we have already mentioned, part of the incoming solar radiation is reflected back. What is not reflected is, in part, either absorbed or scattered by atmospheric gases, vapours and dust particles.

Atmospheric gases absorb solar energy. They selectively do so at certain wavelength intervals called absorption bands. Those wavelength regions that are not absorbed are known as transmission bands or atmospheric windows. Most of the lethal, very short range of wavelengths (Gamma rays, X-rays and UV rays) is absorbed by oxygen, nitrogen and ozone. Carbon dioxide and water vapour, in contrast, absorb the red and infrared range. Due to the reflection, scattering, and absorption of radiation, the quantity of solar energy that ultimately reaches the ground is much reduced in intensity. The amount of reduction varies with the radiation wavelength, and depends on the length of the path through which the solar radiation traverses the atmosphere. It also varies with such factors as latitude, season, cloud coverage, and atmospheric pollutants.

On the other hand, the ground reflects, absorbs and emits light. Reflection from the ground is primarily visible light. The relatively small amount of energy radiated from the Earth at an average ambient temperature of 17°C at its surface consists of infrared radiation with a peak concentration at 970 nm (nanometres, 1nm= one billionth of a metre). This invisible radiation is dominant at night.

The graphs on Figure 2.13 show how the atmosphere filters both the incoming solar radiation and the surface emitted radiation. The top panel displays the transmitted wavelength ranges while the middle panel displays the complementary image. We can observe that the atmosphere absorbs chiefly in the UV and infrared-thermal domains. In the third panel we find the atmosphere chemical constituents liable for that effect. Surprisingly for many, water vapour is the major absorbent of thermal radiation and the chief responsible player for the warming of the atmosphere. Far behind, come, in this order, carbon dioxide, methane and nitrous oxide.

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Figure 2.13 Absorption bands in the Earth’s atmosphere created by greenhouse gases and the resulting effects on both solar radiation and upgoing thermal radiation. (Top) Solar (red) and soil (blue) radiations transmitted by the atmosphere. (Centre) Percentage of radiation captured by the atmosphere. (Bottom) Contribution of greenhouse gases to absorption and scattering of radiation. Credit: Robert Rohde. Source: Global Warming Art, http://www.globalwarmingart.com/wiki/File:Atmospheric_Transmission_png(accessed 6 March 2012).

If water vapour accounts for most of the atmospheric warming, why is there so much noise about the anthropogenic emissions of carbon dioxide? The atmosphere warming is a natural effect. It naturally warms, but also cools by radiating back to outer space. When both regulatory mechanisms are in balance, the atmosphere reaches a more or less constant temperature. This has permitted the existence of life on earth because, without an atmosphere, temperatures would mimic conditions on the Moon: a maximum surface temperature of 123 ºC and a minimum of –233 ºC. The problem comes when there is an imbalance between warming and cooling processes in the atmosphere. This is just what is presently occurring. Carbon dioxide which, although certainly less active than water vapour as a warming agent, is increasing its concentration at a very high rate. It is enough to disrupt the warming-cooling balance (water vapour concentration remaining constant) and thus be accountable for the temperature rise.

2.2.8 Greenhouse gases.

12

All gases which absorb infrared radiation are greenhouse gases: water vapour, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3) and man-made

molecules such as fluorinated gases or halocarbon compounds. The efficacy with which a gas contributes to the greenhouse effect depends on both its concentration in the

atmosphere and its capacity to absorb infrared light.

12 Popular, expressions such as ‘greenhouse effect’ or ‘atmosphere acts as a blanket’ are actually misnomers. Usage of words like ‘greenhouse’ or ‘blanket’ is partly incorrect. The air inside a greenhouse certainly heats up but the great difference with the atmosphere is that air in the former is trapped and convection prevented. A blanket is an insulator, the purpose of which is to block energy transfer across it, which is of course not what happens in the case of the atmosphere. We shall nevertheless invoke here the conventional usage of the

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Figure 2.14 (Top) Relative contributions of some of the natural components of the atmosphere to radiative forcing (i.e. the contribution to the greenhouse effect). (Bottom) Man-made contribution to radiative forcing for main greenhouse gases; percentages denote the contribution for each type of gas – not to be confused to the actual concentration for that gas in the atmosphere. (Figure by Victor Fairén)

Figure 2.14 displays the figures for the contribution of each of these agents. The top chart refers to the natural contribution. Here, water vapour accounts for more than half of the total, while other gases (including naturally present carbon dioxide) account only for 30%. The bottom chart displays the added contributions, which result from human activities. Carbon dioxide collects the major part. Currently, methane amounts to 16.5% of the effect, which is a share in clear disproportion to its actual concentration. Its impact, molecule by molecule, is several times more powerful than that of carbon dioxide and it represents a potentially very dangerous source given the huge quantities of trapped methane that can be released if temperatures continue to rise.

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Carbon dioxide (CO2)

Figure 2.15 (Top) Atmospheric concentrations of important long-lived greenhouse gases over the last 2,000 years. Source: IPCC Fourth Assessment Report, chapter 2: Changes in Atmospheric Constituents and in Radiative Forcing:

http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-2–1.html(accessed 6 March 2012). (Bottom) Atmospheric carbon dioxide concentrations as directly measured at Mauna Loa, Hawaii. This curve is known as the Keeling curve, after Charles Keeling, who started systematic instrumental measurements at Mauna Loa Observatory. Red line reflects seasonal variations (see insert) while blue line stands for trend. Credit: Robert Rohde. Source: Global Warming Art,

http://www.globalwarmingart.com/wiki/File:Mauna_Loa_Carbon_Dioxide_png(accessed 6 March 2012).

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CO₂belongs to the natural carbon cycle, whereby most plants absorb carbon dioxide and animals release it, and has always been a constituent of the atmosphere. Its atmospheric concentration has been increasing for more than a century due to the anthropogenic alteration of the natural carbon cycle, being today at a level never attained during the last 650,000 years13.The graphs of the evolution of CO₂in two separate periods, 0–2005 (top) and 1960–2010 (bottom), are represented in Figure 2.15. We notice the rather dramatic upsurge since the 19th century. Pre-industrial CO₂levels averaged 280 parts per million; as of 2010, the level is at 390 ppm and steadily climbing, representing an increment of 39% in less than 200 years. The figure is even more striking when seen in the perspective of the last 400 000 years. The top panel of Figure 2.16 shows the time-series of CO₂concentrations for the last 400 thousand years, reconstructed from samples taken from trapped air bubbles in the Vostok ice core in Antarctica.14_{The top value for the concentration of carbon dioxide}

for the whole period analysed varies between 180 ppm at the coldest episodes and 280–300 ppm at the warmest, which is well under the present figure. Upon comparing top and bottom panels in Figure 2.16 we notice the correlation between carbon dioxide concentrations and temperatures.15

Figure 2.16 Historical trends in carbon dioxide concentrations and temperature, on a geological and recent time scale. Cartographer: Philippe Rekacewicz, UNEP/GRID-Arendal. Source:

http://maps.grida.no/go/graphic/temperature-and-co2-concentration-in-the-atmosphere-over-the-past-400–000-years(accessed 6 March 2012).

Seasonal variations observed on the annual cycle in the bottom chart of Figure 2.15 are due to the growth of vegetation during the warm season, which retrieves through photosynthesis CO2from the atmosphere, while the decomposition of that same

vegetation during the cold season releases CO2. Notice that these oscillations for the CO2

concentration follow the seasonal fluctuations of the Northern Hemisphere because of the higher concentration of continental landmasses in that hemisphere.

13 In a recent paper (Hönisch et al., Science Vol. 324. no. 5934, pp. 1551 – 1554) the authors show that peak CO2levels over the last 2.1 million years averaged only 280 parts per million.

14 The Russian Vostok station in East Antarctica yielded the deepest ice core ever recovered, reaching a depth of 3,623 m in 1998.

15 Global climate and CO2are correlated, as inferred from Figure 2.16, but the causes of this correlation in

the case of the Pleistocene ice ages are unknown. Temperature modifications are believed to have their cause in switches in insulation due to orbital cyclic variations – see Section 2.3.2.2. An ensuing mechanism of CO2

release or sequestration by warmer or colder oceans, respectively, seemed to be at work here. There is much yet to learn about this quoted ocean uptake.

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For the greatest part, this unrelenting accumulation of carbon dioxide is the result of two hundred years of the massive combustion of fossil fuels16_{– essentially, coal, oil and gas}

– which were produced eons ago as the result of the burial by sediments of plants and animals in anaerobic conditions. This CO2accumulation is happening at an ever

increasing pace: in the decade 1998–2008 carbon emissions rose by an average of 2.5 percent – nearly four times as fast as in the 1990s [Le Quéré et al., 2009].

It has been estimated that approximately fifty percent of all anthropogenic carbon dioxide ever emitted has been permanently removed from the atmosphere. In spite of that, its atmospheric concentration continues to rise. In the short term, the efficiency of oceans as CO2sinks may be impaired by global warming, as the gas solubility in water decreases

with temperature.

Figure 2.17 IPCC. The global carbon cycle for the 1990s, showing the main annual fluxes in GtC/y (Gigatonnes of carbon per year): pre-industrial ‘natural’ fluxes in black and ‘anthropogenic’ fluxes in red. At the end of 1994 the net cumulative atmospheric carbon content of anthropogenic origin amounted to 165 GtC, to be added to the estimated 597 GtC present in 1750, before the Industrial Era. Source: IPCC Fourth Assessment Report, chapter 7, Couplings Between Changes in the Climate System and Biogeochemistry. http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch7s7–3.html#7–3–1(accessed 6 March 2012).

Figure 2.17 displays the global carbon cycle in the early 1990s as estimated by the IPCC in its Fourth Assessment Report [2007].17_{The graph reveals (arrows in red) the effect of}

16 The two major sources are CO2 emissions from fossil-fuel combustion and industrial processes, and the CO2 flux from land-use change, mainly deforestation (see van der Werf et al., Nature Geoscience, Vol. 2, pp. 737-8).

17 Actually, carbon budget estimates depend on authors. For example, Houghton [2009, p. 40] provides the range (0.5 to 2.7 GtC yr-1) for land-use change (LUC) emissions -from factors such as deforestation, land use data, fire observations, assumptions on carbon density of vegetation (Le Quéré et al., 2009) - with a mean estimate of 1.6 GtC yr-1, and a range (0.9 to 4.3 GtC; mean 2.6 GtC yr-1) for land sink (LS). Le Quéré et al. [2009] give the value 1.5 ± 0.7 GtC yr-1 for net LUC CO2 emissions for the period 1990-2005 and a mean land uptake (LS) of 2.2 ± 0.4 GtC yr-1 for the period 1990-2000. Sarmiento et al. [2010], upon comparing different source authors, evaluate the net land carbon sink (LS-LUC) at a mean value of 1.15 GtC yr-1 between 1989 and 2007. Despite this variability in the sources, these authors find agreement between different models in showing a clear tendency towards an increase of net land carbon sink values during the period 1960-2005, which, they conclude, may be attributed to a shift to a higher overall land uptake rate during that period. The reasons for the jump in LS values remain unclear .

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human interference on natural values for fluxes18_{(arrows in black) and reservoir contents} (see boxes in graph). As a result of combustion of fossil fuels, 6.4 Gigatonnes19_of carbon equivalent (GtC) per year, on average, were dispersed into the atmosphere during those years. Land use change (mainly deforestation) contributed to an additional input of 1.6 GtC/y. This total flux of carbon to the atmosphere was partly counterbalanced by the absorption of atmospheric CO2by both land and oceans. An estimated 2.2 GtC/y were

absorbed by vegetation (growth) and soil (detritus). Oceans were estimated to take 1.6 GtC/y. All processes summed up, there was a net atmospheric gain of 3.8 GtC/y (8-4.2). The figures in boxes (Fig. 2.17) give an idea of the relative impact of human activities since 1750. During the period 1750-1994, and according to IPCC estimates, 244 GtC of fossil carbon and 140 GtC from land-use changes, were transferred to the atmosphere, totalling 384 GtC. Part of this huge amount of carbon was reabsorbed by land and soil (101 GtC) or was dissolved in surface and deep oceans (18 and 100 GtC, respectively). Consequently, the atmosphere still retained at the end of that period 165 GtC (384-101-18-100), product of human activities. This amount is to be compared to the natural atmospheric carbon content in 1750, which was estimated by the IPCC to total 597 GtC. It means a net 28% increase in two hundred and fifty years.

An understanding of CO2exchanges between land and atmosphere is crucial in order to

perfect the description of the carbon cycle and therefore make better predictions on the future levels of this greenhouse gas [Reich, 2010]. In the context of mitigation, much has been debated on the issue of CO₂fertilisation, an intensification of vegetation growth due to higher carbon dioxide atmospheric levels. It may be one of the causes behind the gains in net land carbon sink values, although field studies are far from conclusive and the debate is still open. Apparently, vegetation should thrive in enriched carbon dioxide conditions as CO2is one of its primary resources. It is nevertheless not as simple as one

might think. Net Primary production (NPP), which quantifies the amount of carbon dioxide fixed by terrestrial vegetation, showed the expected intensification in the 1980s and 90s [Nemani et al, 2003], although it is not clear if that happened in response to higher atmospheric carbon contents . However, in the first decade of the 21st century, the warmest on record, the trend has been inverted [Zhao and Running, 2010]. Large areas of South America, South Africa and Australia are the most affected. The phenomenon seems to be related to the unfavourable rainfall conditions which have dominated the decade in the Southern Hemisphere, and possibly fires and the substitution of large areas of tropical forest (in Brazil and South East Asia) by cash crops. Although the

phenomenon needs to be confirmed by additional monitoring and studies, this reversal should be a clear warning to those who count on carbon sequestration by vegetation for counteracting fossil fuels burning.

In summary, roughly speaking, of the estimated 244 GtC emitted into the atmosphere since the beginning of the industrial era, some 165 GtC have settled there (a 27%

increase with respect to pre-industrial levels). Meanwhile, current reserves of fossil fuels are estimated to be of the order of ten times the amount already gone into fumes.

Although most of them are either unreachable or too expensive to exploit in present conditions, we cannot envisage a good prospect for the future if the world persists in its unabated dependence on fossil fuels.

18 The word ‘flux’ (plural ‘fluxes’) appears many times in this chapter. Here and in Figure 2.17 it refers to the amount of carbon dioxide incident on an area (land surface, ocean, atmosphere, etc.) in a given time. It can also refer in a similar way to the amount of radiation/energy falling on an area in a given time. 19 A Gigatonne is equal to one billion (1,000,000,000) tons.

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Methane and nitrous oxide

Methane (CH4) is second to carbon dioxide in accounting for the present global warming,

with nitrous oxide (N2O) in third position. Accurate measurements yield increasing

atmospheric concentrations for these two gases (see top plate in Figure 2.15), with little geographic variation. The rise with respect to pre-industrial levels is spectacular. This is particularly disturbing in the case of methane because no satisfactory explanation has yet been found.

Methane has a concentration two hundred times smaller than that of carbon dioxide20_but

it accounts for nearly 17 percent of the additional (anthropogenic) warming effect. Its effect, molecule for molecule, is thus more intense than that of carbon dioxide. This apparently paradoxical effect can be explained by the capacity of this gas to absorb infrared radiation due to its chemistry. In return, its lifetime in the atmosphere is much shorter than that of carbon dioxide – a few years compared to scales of the order of centuries to millennia [Archer et al., 2009].

Methane concentration has risen 150% with respect to pre-industrial times. Half of this amount is estimated to come from human activities. The main sources of methane are to be found in anaerobic fermentation (i.e. in the absence of air), cattle, termites, fossil fuels burning and natural gas leakages. Most anaerobic fermentation takes place in flooded areas. Swamps, ponds, rice fields and peat-bogs are thought to release annually some 300 million tonnes of methane. Ruminants are also an important source of methane – 100 million tons per year. In the southern hemisphere, termites play an active role in producing methane (15–30 million tonnes annually). Finally, some 300–450 million tonnes annually are freed as the result of natural gas leakages, fossil fuels burning, deforestation and fermentation in dumping sites.

Nitrous oxide (N2O)21is the third most important contributor to radiative forcing. It is a

natural component of the Earth’s atmosphere. In pre-industrial times, the concentrations were about 270 ppbv (parts per billion by volume). Bacteria in soils and sediments are the most important source of atmospheric N₂O. They produce N₂O as a by-product of nitrification and denitrification, which are important processes in the natural nitrogen cycle. N2O escapes from the soils and sediments to the atmosphere, and is eventually

broken down in the stratosphere, more than 10 km above the Earth’s surface, by ultraviolet radiation. The atmospheric lifetime of N2O is relatively long (more than a

century) making it a powerful greenhouse gas. It has a much larger warming potential than CO2and CH4. One kg of N2O emitted absorbs 296 times as much infrared radiation

as one kg of CO2emitted (Global Warming Potential over 100 years [IPCC, 2007]).

The atmospheric concentrations of N2O have been increasing for about a century to 319

ppbv in 2005. This increase is largely associated with increased emissions from soils associated with food production for a growing world population [Kroeze et al., 1999; Mosier et al., 1998]. Increased use of fertilisers in agriculture leads to increased levels of reactive nitrogen in soils and sediments. This stimulates bacteria to produce more N2O in

soils and sediments than under natural conditions. Not only is agricultural land a source of anthropogenic N2O, but also natural soils and aquatic systems, where levels of reactive

nitrogen have increased as a result of fertiliser losses and atmospheric N deposition [Syakila and Kroeze, 2011]. Industrial sources of N2O add to this, but are of minor

importance.

20 It is given in parts per billion (ppb) on Figure2.15 instead of parts per million (ppm) 21 We thank Dr. C. Kroeze for authoring the contribution on N

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Water vapour

Water vapour is by far the most important greenhouse gas. It produces more atmospheric warming than any other gas, even if a water molecule is less efficient in absorbing infrared radiation than a CO2molecule. The equilibrium vapour pressure of liquid water,

and correspondingly the maximum amount of water in the atmosphere, augments exponentially with temperature. If this is a well-known physical fact, the consequences are not so clear. As a result, the infrared thermal radiation redirected by water vapour will consequently be boosted by the global warming produced by other gases (and itself), and temperatures will rise even more. But, on the other hand, cloud formation will be

stimulated in a vapour rich atmosphere, and clouds are known to have a mixed effect on infrared thermal radiation. Water in the form of liquid droplets absorbs infrared thermal radiation as well. Depending on their type, clouds behave differently with respect to incoming solar radiation. Putting it in very simple terms, it can be said that low altitude clouds do usually reflect solar radiation, and allegedly cool the atmosphere, while high altitude clouds, on the contrary, show the opposite behaviour and warm it. However, our understanding of clouds and their effect on global warming and climate is still very incomplete. It is not then evident if a higher atmospheric vapour content will have a positive or a negative influence on global warming.

2.2.9 Climate variability and forcing agents

Physical climate models are a kind of compendium of the present body of knowledge on climate and its dynamics. Consequently, we shall ultimately have to resort to those models in order to fit together the different pieces of the climate puzzle. We will deal more profoundly with the nature of climate models later, in Section 2.4. In the meantime, we need for our present purpose to consider the factors we know to exert an influence on the long-term global behaviour of the atmosphere, and thus on climate.

In addition to the atmospheric chemical composition, other driving forces have an effect on climate: The amount of solar energy striking the Earth’s surface and its geographical distribution, the distribution of continental landmasses and their drift over time, mountain formation, ocean levels or volcanic eruptions, are all physical realities to be taken into account in a comprehensive picture of climate behaviour and change.

The climate system is defined by the IPCC Fourth Report as: “… an interactive system consisting of five major components: the atmosphere, the hydrosphere, the cryosphere [areas of ice], the land surface and the biosphere [areas of vegetation], forced or influenced by various external forcing mechanisms, the most important of which is the Sun (see Figure 2.18). Also the direct effect of human activities on the climate system is considered an external forcing…”